Method of testing a proximity fuze



March 28, 1961 v 7 METHOD OF TESTING A PROXIMITY FUZE Filed Feb. 12,1953 PATH 0F DIPOLE PEA K Roberi C Lo Vick INVENTOR.

BY Q/Z W ATTORN E Y8 .PEAKLY Fig. 4-

rates METHOD OF TESTING A PROXIMITY FUZE Filed Feb. 12, 1953, Ser. No.336,478

3 Claims. (Cl. 343-175) This invention relates in general to electricaltesting, and more particularly to apparatus for measuring the relativesensitivity of proximity fuzes for ordnance projectiles.

When an ordnance projectile equipped with a proximity fuze is fired froma gun, the oscillator of the fuze generates a continuous radio frequencysignal. As the projectile approaches a reflecting surface, a portion ofthe energy radiated from the fuze is reflected back into the oscillatorcircuit of the fuze. As a result, the radiation load of the antenna ofthe fuze changes and varies through a series of maximum and minimumvalues of increasing magnitude as the reflecting surface is approached.Consequently, the current in the oscillator plate circuit rises andfalls. A resistor in the oscillator plate circuit translates this riseand fall of plate current into a correspondingly fluctuating voltage.This voltage is applied through an amplifier to actuate a thyratron whenthe projectile comes within lethal range of the reflecting surface. Thethyratron acts as an electronic switch to operate an electric detonatorcalled a squib or booster. The blast from the booster operates anelectrical detonating fuze which sets off the explosive charge in theprojectile.

The change in the radiation loading of the fuze antenna, which in turncauses the oscillator plate current to rise and fall, is a primaryfactor in the ability of the fuze to function. The character of thefluctuations of plate current depends upon several factors, includingthe size and reflectivity of the target, oscillator frequency, velocityof approach, angle of approach, and the characteristics of theoscillator itself. However, if proximity 'fuze oscillators are subjectedto standard conditions of antenna loading in a stable outdoorenvironment, and the electrical response of the oscillator to a standardchange in such conditions is observed, the measurements will be anindication of the relative sensitivity of proximity fuze.

Such measurement, known as the pole test, is the primary method utilizedheretofore to measure the sensitivity of the oscillator of proximityfuzes. In the pole test changes in the plate current of the oscillatorare observed when a fuze mounted in a projectile shell is movedperpendicular to a large reflecting surface which is parallel to theaxis of the shell. A proximity fuze is assembled within a test shellconforming in contour to the projectile shell in which the fuze is to beused. The test shell is supported above a metallic screen laid upon theground by means of a suitable mechanism so that it can be raised andlowered, and the changes in the plate current of the oscillator areobserved when the radiating shell is moved through the standing wavepattern caused by reflection from the screen. If the shell is slowlyraised from the ground level, the oscillator plate current will be foundto go through maxima and minima at certain distances above the ground,depending upon the frequency of the oscillator. For a grid detectoroscillator a maximum of current will be atent found approximately atabove ground and a mini mum at /8), the next maximum will be at /s7\ andthe next minimum at P/sh. As the distance from the reflecting surfaceincreases, the eifect gradually dies out; The resulting steady platecurrent is equal to the plate current which would be that of free spaceloading, i.e., the value existing in the absence of any reflectingsurface.

The shell can be considered as a small antenna radiating energy in theform of an electric field and a magnetic field. A portion of this energyis reflected when the antenna is brought near a reflecting surface suchas the earth. The resultant of the incident and reflected electric fieldhas nodes separated by one quarter wave length from the nodes of theresultant of the incident and reflected magnetic field. The total energyradiated for any given position of the oscillator is the cross prod: netof the two and causes a minimum in oscillator plate current to occur at/8 wavelength, a maximum at wavelength, a minimum at wavelength, etc.

It is possible to indicate the sensitivity of the oscillator of aproximity fuze as a given change in plate current occurring betweenspecified peaks and dips such as, for example, between 1% and 1%wavelengths above ground. However, the actual height above ground atwhich these points will occur will vary with frequency, so thatsensitivity designated in this manner gives no information as to thesensitivity at a definite height. For this reason, the sensitivity ofthe oscillator of a proximity fuze has been defined as that at 15 feetabove ground. The plate current is measured at a peak and a dip nearfifteen feet and the result corrected to fifteen feet. The measuredplate current I flows through a resistance R in the plate circuit of theoscillator, and the voltage developed is I XR. Since the measuredcurrent is peak to peak value, and only the positive half of the voltageis required to fire the thyratron which detonates the booster, thevoltage I R is divided by 2 /2 to obtain the sensitivity of theoscillator in root mean square amplitude. The distance of 15 feet isthus arbitrarily chosen at the point at which measurements will be takenstatically to determine the relative sensitivity of proximity fuzes.

Because the pole test must be performed in a large flat area that isfree from objects that might reflect the radiated waves, a location mustnormally be selected away from an urban area. The necessity of movingthe fuzes from a laboratory to a distant field location for checkingsensitivity is inconvenient and hampers experimental development. It isobvious that the inexpediency of having to send fuzes to a fieldlocation for measurement of sensitivity after every experimental changein construction necessitates a laboratory means of checking sensitivity.Such a secondary standard of measuring sensitivity would provide anaccurate means of checking the basic testing done at the field pole.

Measurements of sensitivity taken by the pole test are particularlyunreliable under conditions of inclement weather, particularly when snowcovers the ground screen. The effect of absorption of the radiatedenergy by snow or moisture are not fully understood. The desirability ofhaving a method to provide accurate measurements at other than the poletest site should be readily apparent.

It is an object of the invention to provide a convenient, accurate andrapid method of and apparatus for dynamically measuring the oscillatorsensitivity of proximity fuzes.

This and other objects are obtained by assembling a proximity fuzewithin a specified vertical test shell and moving a vertical dipoleantenna having a length apea -me proximately one half the wavelength ofthe energy radiated by the" proximity fuze toward the shell from a pointremote from the shell through one position wherein the energy reradiatedfrom the dipole antenna producesa'minimum of plate current'in theoscillator :of the proximity fuze to a second position wherein theenergy reradiated from the dipole causes a maximum of oscillator platecurrent. The center of the reflecting dipole antenna is in the samehorizontal plane as the electrical center of the radiating dipole of theproximity fuze. The amplitude of the observed ripple signal voltagevariation between maxima and minima of oscillator plate current is anindication of the electrical sensitivity of the oscillator of theproximity fuze. This method of measuring the relative sensitivities ofproximity fuzes willbe referred to in the specification as the rotatingdipole method.

' Referring now to the drawings wherein like reference numerals;designate like parts:

Fig.1 is a schematic perspective view of one embodi- 'nert of theinvention in which two dipole antennas are use Fig. 2 is a schematicperspective view of the preferred embodiment of the invention in which asingle "rotatable dipole is continuously moved into and out of theradiated field;

' Fig. 3 is a diagrammatic plan view of the embodiment shown in Fig. 2;and

Fig. 4 graphically illustrates the changes in plate current withrespectto height above ground.

To'produce 'a system of dipoles which are positioned by distance andoriented on a rotating rod such that by rotating the rod through ninetydegrees, the net effect is a measure of the sensitivity is disclosed inFig. l. The fuze is assembled in a projectile casing 11, the casingbeing positioned on an insulating support 12. A vertical dipole antenna13 is mounted on one end of rod 14 and a horizontal dipole antenna 15 ismounted on the other end thereof. Rod 14 is rotatably mounted in bearing16 on post 17 which is mounted on cart 18. Each dipole antenna includestwo metallic rods, rods 19 in antenna 13 and rods 20 in antenna 15,which slidably telescope into the tubular portions thereof. The rods 19and 20 can be extended to a position in which the overall lengthisapproximately twice that of the cylindrical portion. This permits theadjustment of the length of the dipole antennas to half the wavelengthof the waves radiated from fuze 10. Antennas 13 and 15 are separatedby'a distance such that by rotating rod 14 through 90 degrees the dipoleantennas are alternately coupled, this distance being approximately onequarter wavelength and producing a maximum and a minimum in platecurrent. With this arrangement dipole antennas 13 and 15 are alternatelycoupled thereby producing a minimum and maximum in plate current. thenet eifect being a measure of sensitivity and describing the entirecurve of variation.

' In the preferred embodiment of the invention illustrated in Fig. 2 ofthe drawings, a proximity fuze 10 is assembled in projectile casing 11,casing 11 being positioned vertically on insulating support 12. A singledipole antenna is mounted in a vertical position at the end of rotatablehorizontal arm 26 constructed of insulating material. The dipole antenna25 consists of a' hollow metallic tube having two metallic rods 27slidably telescoped into the ends thereof. Rods 27 can be withdrawn to aposition wherein substantially the full length of each rod extendsbeyond the ends of the antenna tube so that the length of antenna 25 canbe increased to approximately twice the length of the tube. This permitsthe adjustment of the length of dipole antenna 25 to half the wavelengthof the waves radiated from proximity fuze 10. The horizontal arm 26 isattached to the upper end of the vertical shaft 28 of a motor 29 whichmay be mounted below the surface of the floor upon which support 12rests to prevent reflection of the energy radiated by proximity fuze 10,in the event shaft 28 is fixed in position with respect to fuze 10, ormay be mounted on cart 18 and shielded to prevent reflection of theenergy radiated by fuze 10. The motor 29 isutilized to rotate the dipoleantenna 25 at the end of horizontal arm 26. The circular path traced bythe dipole antenna 25 during rotation of horizontal arm 26 is shown inFig. 3. In both embodiments of the invention, rod 14 and arm 26 arearranged in the radiating or electrical plane of fuze 10.

The radiation pattern from the dipole of a proximity fuze isapproximately at right angles to the plane of the axis of the fuze andencircles the 'fuze completely. The shape of the radiation patternresembles a doughnut with the axis of the fuze passing through itscenter. When the axis of a fuze is vertical, the radiationisapproximately equal in all directions in the horizontal plane while thevertical radiation along the direct line of the axis is substantiallyzero. The wave radiated from a vertical proximity fuze 10 issubstantally vertically polarized.

A receiving antenna is able to abstract energy from a passing radio waveas a result of the voltage that the magnetic flux of the wave induces inthe antenna. The value of the induced voltage is proportional to thefield strength, the cosine of the angle between the plane ofpolarization of the wave and the wire in which the voltage is induced,and the cosine of' the angle between the wave front andthe direction ofthe wire. Thus a horizontal wire in the field radiated from a verticalproximity fuze will have a substantially zero voltage induced therein,while the voltage induced in a vertical wire will be a maximum.

The dipole antenna 25 absorbs power and reradiates it in a mannersomewhat analogous to a parasitic reflector or a parasitic director.When the field produced by the dipole antenna of proximity fuze 10 cutsdipole antenna 25, it induces a voltage therein which in turn produces afield which cuts the dipole of proximity fuze 10. The dipole antenna 25is not positioned at a fixed fractional wavelength from a driven antennain the manner that a parasitic reflector is utilized, but in measuringoscillator sensitivity the dipole antenna 25 is moved from a positionwherein the reradiated power cutting the dipole of proximity fuze 10produces a minimum in oscillator plate current through a second positionwherein the reradiated power causes a maximum of plate current. Thedipole antenna 25 thus reflects the energy radiated from the dipole ofthe proximity fuze somewhat analogously to the manner in which standingwaves are set up in a closedend transmission line by reflection from theshort-circuited end thereof. A portion of the energy radiated byproximity fuze 10 is reflected back by dipole antenna 25 into theoscillator circuit of the fuze. As a result, if a vertical dipoleadjusted to approximately one half the wavelength of the radiated energyand not connected to a power source is positioned on movable cart 18 andadvanced from a distant point toward a proximity fuze, the radiationload as seen by the dipole antenna of proximity fuze 10 changes, varyingthrough a series of maximum and minimum values of increasing magnitudeas the dipole approaches proximity fuze 10. If in advancing a dipoleantenna toward fuze 10 on movable part 18, a point is found wherein thereradiated energy causes a node in oscillator plate current, then thedistance between the'axis of casing 11 and motor 29 is adjusted untilshaft 28 is approximately at this point and motor 29 is controlled so asto rotate horizontal arm 26 slowly. At this point the plate current ofthe oscillator will vary from a minimum to a maximum and from a maximumto a minimum in the same manner as when the fuze is raised from andlowered toward the ground screen in the pole test.

Fig. 4 illustrates the changes in plate current observed by the poletest when a fuze, mounted in a test shell, is moved perpendicular to alarge, reflecting surface is parallel to the test shell axis. Such adiagram is spoken of as a ripple pattern. The changes in plate currentare made readily measurable by bucking out the steady plate currentof-the oscillator, leaving only the plate current fluctuations to beobserved on a sensitive meter.

As described hereinbefore, the sensitivity of the oscillator asdetermined by the pole test has been arbitrarily defined as a givenchange in plate current at a definite height above ground in order toallow comparison of the relative sensitivities of proximity fuzeoscillators. The plate current variations observed by the rotatingdipole method when antenna 25 is rotated by motor 29 also provides adirect indication of the relative sensitivity of a proximity fuzeoscillator. Sensitivity measured by the rotating dipole method is bestdefined as the root mean square amplitude of that cycle of thealternating current ripple signal voltage which encompasses a point on aplane at a fixed distance from fuze as dipole antenna 25 is moved towardthe shell. Experiment has shown that the position of the dip and thepeak are not an exact number of A; Wave lengths from the axis of theshell, and that the position of the peak and dip vary somewhat with thelength of dipole antenna 25. Although changing the dipole length variesthe position of the dip and peak, the total voltage excursion remainsapproximately constant. This provides a rapid and re liable method ofchecking sensitivity measured by the pole test.

Although the rotating dipole method can be carried out in a room thatwill not reflect energy radiated by proximity fuze 10, the most accurateresults are obtained if the apparatus is located on the roof of abuilding where no reflection from side walls is possible. Also, toobtain more accurate results, the casing should be placed at aconvenient distance from the floor or roof level. Any horizontalmetallic objects, such as pipes that might act as reflectors of theenergy radiated from a vertical proximity fuze are in a plane where suchmetallic objects will have substantially zero voltage induced thereinand, consequently, will re-radiate only a minimum of power.

To utilize the method of the invention to check the readings ofproximity fuze sensitivity measured by the pole test, it is desirable toadjust the distance between the axis of casing 11 and the shaft 28 sothat the rotation of dipole antenna 25 will produce plate currentvariations which numerically approach the readings obtained by the poletest. The sensitivity of the oscillator of a proximity fuze has beenarbitrarily defined as the amplitude of the cycle of ripple voltagesignal which encompasses a point fifteen feet from the ground screen asthe shell is moved perpendicular to the ground screen. If a dipoleantenna is supported upon movable cart 18 and advanced from a remotepoint toward the proximity fuze 10, the oscillator of fuze 10 will varythrough a series of maxima and minima of increasing amplitude as dipole25 approaches casing 11. The amplitude of one cycle of oscillator platecurrent variation will approximate within five percent the amplitude ofplate current variations obtained by the pole test at fifteen feet fromthe ground screen, e.g., the oscillator plate current variation may havebeen found to be 92.2 rnicro-amperes at feet by the pole test, while88.3 micro-amperes swing may have been the closest numerical valuemeasured between a peak and a dip as the dipole approaches casing 11.The oscillator plate current variations measured by the rotating dipolemethod will never read exactly the same as the pole test measurements. Afactor of proportionately must always be used to correct the readingsobtained by the rotating dipole method, i.e., it will be necessary tocalibrate the apparatus embodying the invention to the pole test.However, once the rotating dipole is calibrated for a given frequencyrange, sensitivity measurements can be made directly without thenecessity of moving the dipole from a remote point toward the proximityfuze to establish the cycle wherein the amplitude of the voltage swingmost closely approximates the pole test measurement of sensitivity. Oncethe position of the motor shaft 28 is fixed for any given range offrequencies it is only necessary to multiply the plate current variationobtained by the rotating dipole method by the constant ofproportionality obtained in the calibration to obtain a reading directlyindicative of the sensitivity measured at fifteen feet by the pole test.

A tolerance of plus or minus a few megacycles in the frequency ofoscillation is usually provided in the production of proximity fuzeoscillators. If the length of dipole 25 is once adjusted for the meanfrequency of the allowable spread of oscillator frequency, the dipolesystem will give repeatable results within the ability of the field poleto reproduce. As dipole 25 is slowly rotated by motor 29, the oscillatorplate current of proximity fuze 10 will vary from a minimum, or dip, toa maximum, or peak, and from a peak to a dip in a continuous motion. Thereadings of plate current variation when divided by 2 /2 and multipliedby the factor of proportionality obtained during the calibration of thedipoles will give a direct indication of pole test sensitivity. It isnot necessary to vary the length of dipole antenna 25 for each dilferentproximity fuze 10 which is inserted within the shell casing 11 for checkof oscillator sensitivity. If dipole antenna 25 is adjusted for the meanfrequency of a normal production'run of frequencies, the dipole systemembodying the method of the invention will give repeatable resultswithin the ability of the pole test to reproduce. The criticalness ofthe length of dipole 25 is reduced if the dipole is relatively thick.Increasing the thickness decreases the Q of dipole 25, so that thelength of the dipole is particularly unimportant when a relatively thickdipole antenna is utilized.

As described hereinbefore, the oscillator sensitivity of a proximityfuze is arbitrarily defined at a distance of fifteen feet from a groundscreen. It is possible to utilize this apparatus as a primary standardfor measuring oscillator sensitivity of proximity fuzes. This is bestaccomplished by defining oscillator sensitivity as the change in ripplesignal voltage occurring between specified peaks and dips such as, forexample, between a dip when the distance between reflecting dipoleantenna 25 and proximity fuze 10 is 2 AM and a peak when this distanceis 2 /s Oscillator sensitivity indicated in this manner is not in termsof pole test measurements but directly in terms of ripple signal voltagevariations occurring when a reflecting dipole is moved between specifiedpeaks and dips. In using a reflecting dipole antenna in this manner as aprimary standard, it is unnecessary tochange the length of dipoleantenna 25 or the distance between the dipole and fuze it) for eachmeasurement of sensitivity. The length of antenna dipole 25 is adjustedfor the mean frequency of the allowable tolerance spread of frequenciesin a manner similar to that described when the dipole method is used asa secondary standard for the pole test. The distance between fuze 10 andshaft 28 is adjusted so that reflecting dipole antenna 25 in rotating atthe end of the horizontal arm 26 moves through positions causing thepeak and dip in terms of which the oscillator sensitivity has beendefined, e.g., through a point 2 AM from fuze 10 causing a dip in theoscillator plate current and a second point 2%)\ from fuze 10 cansing apeak in plate current. The readings of oscillator plate current for eachprojectile tested (when converted to voltage indications by multiplyingby the resistance on the oscillator plate circuit and dividing by 2 /2)are obtained by connecting an electronic voltmeter across the platecircuit of the oscillator in a well-known manner and are thus directmeasurements of the relative sensi tivity of proximity fuzes and arerepeatable within the limits of accuracy that the field pole test canreproduce.

Whenever it is necessary to calibrate the diploe method for a new rangeof frequencies, it is necessary to employ a movable cart to trace. theripple pattern just as in the original calibration." Moving reflectingdipole 25 on cart 181mm "a remote point toward radiating fuze 1% willcause the oscillator plate current to vary through the ripple patternand allow the determination of the distance between fuze 1G and shaft 28which will permit dipole antenna 25 to move through the specified dipand peak when rotated at the end of the horizontal arm 26, e.g., througha point 2%)\ from the axis of casing 11 causing a'dip in oscillatorplate current and also through a second point 2%% from fuze causing apeak in oscillator plate current. If the length of dipole antenna 25 isadjusted to the mean frequency of the range of frequencies allowable inproduction, direct readings of the relative sensitivity of oscillatorswithin this frequeneyrange can be made directly by the rotating dipolemethod. 'Iclaim: a j

1, The method of measuring the sensitivity of the oscillater of aproximity fuze which is adapted to radiate a radio frequency wavegenerated by said oscillator and substantially polarized in the plane ofthe axis of said fuze when said fuze is electrically actuated whichcomprises assembling the proximity fuze in a vertical projectile shellin a locationremote from vertical obj-ects that reflect radio frequencyenergy, positioning the center of a vertical dipole antenna .insubstantially the same hori zont al plane as the electrical center ofthe proximity fuze, electrically actuating the fuze, adjusting thelength of said dipole antenna to approximately one half the wavelengthof the radiated radio frequency energy, moving the dipole into saidradiated wave whereby the dipole will reflect the energy radiated fromsaid fuze and the standing waveresultant or the radiated and reflectedradio frequency energy will cause the plate current of said oscillatorto vary between a series of maxima and minima of increasing. magnitudeas the dipole approaches the fuze, and then measuring the amplitude ofthe fluctuations of oscillator plate current.

' 2'. The method of measuring the sensitivity of the oscillator of aproximity fuze which is adapted to radiate a radio frequency signalgenerated by the oscillator and substantially polarized in the plane ofthe axis of the ime when said fuze is electrically actuated whichcomprises assembling the proximity fuze in a vertical projectile shellin a location remote from vertical objects that reflect radio frequencysignals, mounting a rotatable vertieal dipole antenna with its center insubstantially the same horizontal plane as the electrical center of thefuze and at a distance from the axis of the fuze equal to a number ofoneeighth wavelengths of the radiated radio frequency signal so thatsaid dipole is coupled to said tuze and the standing wave resultant ofthe radiated signal and the signal reflected by said dipole causes amaximum to occur in the plate current of the oscillator of the fuze,mounting a rotatable horizontal dipole an tenna with its center in saidhorizontal plane and in the plane defined by the axis of the fuze andthe center of said vertical antenna and with a separation between thetwo dipoles of approximately one-quarter wavelength of said radiatedsignal, adjusting the length of both dipole antennas to approximatelyone-half of said wavelength,

electrically actuating said fuze, rotating said vertical dipole antennato a horizontal position to uncouple said vertical dipole, rotating saidhorizontal dipole to a vertical position to couple said horizontaldipole to said proximity fuze to cause a minimum current flow in theoscillator plate, and then measuring the amplitude of the cycle ofoscillator plate current from said maximu to said minimum.

-3. The method of measuring the sensitivity or the oscillator of aproximity fuze which is adapted to radiate radio frequency energysubstantially polarized in the plane of the axis of the fuze when thefuze is electrically actuated which comprises assembling a proximityfuze in a vertical projectile shell in a location remote from V61:

tical objects that reflect radio frequency energy, mount-- ing avertical dipole antenna with its center in substan-. tially the samehorizontal plane as the electrical center of the fuze and at the end ofa rotatable horizontal arm of greater length than one-eighth thewavelength of said radio frequency energy, adjusting the length of thedipole antenna to approximately one-half the wavelength of said radiofrequency energy, electrically actuating said fuze,

and slowly rotating said dipole antenna with the axis of.

rotation of said arm positioned approximately a plurality of quarterwavelengths of said energy from the axis of the "iuze whereby thestanding wave resultant of the radiated energy and the energy reflectedby said dipole antenna will cause the plate current of the oscillator ofsaid fuze to vary from a maximum to a minimum and from a minimum to amaximum in one complete rotation of said arm, and then measuring theamplitude of one cycle of the standing wave of the oscillator platecurrent from said maximum to said minimum.

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